The solar-powered production of hydrogen for use as a renewable fuel is highly desirable for the world’s future energy infrastructure. However, difficulties in achieving reasonable efficiencies, and thus cost-effectiveness, have hampered significant research progress. Here we propose the use of semiconductor nanostructures to create a type-II heterojunction at the semiconductor–water interface in a photoelectrochemical cell (PEC) and theoretically investigate it as a method of increasing the maximum photovoltage such a cell can generate under illumination, with the aim of increasing the overall cell efficiency. A model for the semiconductor electrode in a PEC is created, which solves the Schrödinger, Poisson and drift–diffusion equations self-consistently. From this, it is determined that ZnO quantum dots on bulk n-InGaN with low In content is the most desirable system, having electron-accepting and -donating states straddling the oxygen- and hydrogen-production potentials for , though large variance in literature values for certain material parameters means large uncertainties in the model output. Accordingly, results presented here should form the basis for further experimental work, which will in turn provide input to refine and develop the model.